The present invention relates to explosive cords and, in particular, to methods for securing the end portions of such cords to booster-bearing end fittings.
End fittings of the type under consideration are widely used to couple the ends of a length of explosive cord to boosters so that a shock wave propagating through the cord can detonate the boosters to initiate the detonation of a main explosive charge or to perform some other comparable function. A wide variety of end fitting arrangements are used and, of course, various types of explosive cords are employed depending upon the particular function which they are intended to serve. However, most end fittings use a sleeve-like bearing member for receiving an end of the cord and providing a relatively-rigid support through which the explosive core of the cord is communicated with the booster. Also, most fittings use some sort of a coupling member, such as a coupling nut, that fits over the sleeve-like bearing member and itself is adapted to be threaded into a mating receptacle.
The explosive cords, in turn, may include such well-known types as the so-called confined detonating fuze, (CDF) which may or may not be wire braided. Such a CDF generally is formed with a metal-sheathed, explosive core portion and a plurality of external sheathing layers formed of a variety of materials such as plastic, fiberglass and the wire braid. Conventionally, these materials are in a woven form having small interstices provided in the weave. Another type of cord is the shielded mild detonating cord (SMDC) which again employs the metal-sheathed explosive core portion. However, instead of being multi-layered, the core is protected by a metal tube which is spaced from the core portion. In addition to its protective function, the tube confines the detonation products.
As subsequently will be appreciated, the present invention specifically contemplates the attachment of either of these general types of explosive cords to end fittings. Additionally, as will be recognized, the present principles readily can be adapted to most if not all end fitting and cord arrangements. It also should be noted that the terms `cord` and `fuze` presently are considered as functional equivalents.
In providing any end fitting, it obviously is necessary to tightly secure the sleeve-portion of the fitting to the cord so that, when the coupling nut is securely in its mating receptacle, the entire arrangement will be tightly bound together as an integral unit. Further, since air or moisture obviously affects propagation of the shock wave, as well as detonation of the booster, it is most desirable to provide end fitting arrangements which are hermetically sealed. Although these purposes, of course, have been recognized, as far as is known the techniques for securing the end fittings have been limited to such expedients as the mechanical crimping together of the end-fitting components or the use of various potting or adhesive materials. Mechanical crimping, when used by itself, provides a rather questionable and unreliable attachment and it does not provide the requisite hermetic seal. Consequently, many fittings combine both the crimping technique and a potting technique in which a resin is introduced usually for the purpose of binding the exterior surfaces of the cord or the bearing sleeve to the member. However, the binding action achieved by the relatively thin layer of resin is itself rather weak. Further since it attempts to bind the outer surface of the cord to the fitting, there is no assurance that the various components of the cord itself may slip one on the other.
It is therefore an object of the present invention to secure an explosive cord end fitting in such a manner that the components of both the cord and the fitting are integrally bound and locked together in an unusually strong, load-bearing manner as well as a manner which significantly improves the hermetic sealing of the bond.
A further object is to adapt the present principles to a wide variety of explosive cord end fittings.
In general, the objects of the invention are accomplished by providing a void disposed radially inwardly of the coupling member of the fitting and extending from substantially the plane of the interior bore of the coupling member radially inwardly to the sheathing of the explosive core portion of the cord. A thermosetting material which employs a strengthening filler, such as short glass fibers, is directly injected into this void to flow in and around the cord components and integrally bind the components to the bearing sleeve of the fitting. The injection of the materials is accomplished with the coupling member withdrawn from its assembled position so as to expose the void and, of course, the coupling member then is moved axially into its fixed position relative to the sleeve and other fitting components. The void is made as large as possible to provide maximum binding strength. Some means, however, should be provided to restrict the axial length of the void so that the injected material can solidly pack itself into the void. A part of the void is provided by entrance slots or openings through which the material is injected. Preferably, a plurality of slots is employed and the injected material permitted to flow into all of the slots for the purpose of providing locking projections such as will be described.